Based on research done by
the Stanford Research Institute (SRI) to construct a three-frequency
multi-band dipole that would work without any need for cut and try
techniques, we pass on this information in the hope that it will help you
more easily get this type of antenna on the air
quicker.

What they came up
with was much improved method over the old cut and prune technique
seen at the bottom of this page.

They found
that the wires at the center feed point had to be separated by at least 5
1/2 inches vertically and theends separated by
38 inches in the 2 to 18 MHz range.As in any
fan dipole construction, all of the dipoles are connected in
parallel but in the SRI method, the separation between them at the
feed point must be maintained.

By this simple change
they found that you could accurately cut the antenna elementlengths for given frequencies and eliminate the need for
pruning.

In the drawing above, the lowest frequency
antenna is on top and is cut 4% short of the standard 1/2 wave length.
(Length in feet= 0.96 times 468 divided by the operating frequency in
MHz).

The middle frequency antenna (lower in
frequency), is cut for an exact 1/2 wave length. (length in feet= 468
divided by the frequency in MHz)

The highest frequency
antenna is at the bottom and cut for 1% longer than the 1/2 wavelength
(length in feet= 1.01 times 468 divided by the frequency in
MHz)

Compared to the construction effort of a standard multi-band
dipole the only difference is the fabrication of a feed block or center
insulator that is about 12 inches vertically by 3 inches wide, so make
sure this is made of a good insulating material such as Lucite, Bakelite,
fiberglass, or PVC.

The end 38 inches
of separation can be maintained by separate halyards on each element or a
spreader bar with a common halyard.The bandwidth will be at least plus
or minus 2% for a 1.5 to 1 SWR according to Stanford Research
Institute.

Editors note: It is
assumed that this method will only work as described if you are
working with a "3 band" multiband dipole. We do not have information for
use with over 3 bands using the SRI method.If you arrive at a better
method, let us know! See feedback
below.You can chose whichever method of constructing the
multiband dipole using either the method above or use the old cut and try
method below......your choice.We would appreciate
any feedback if you use the newer method
ABOVE!Email n4ujw at hamuniverse.com with your comments to
be added to
feedback.------------------------------------------------------------------------------------------------

Feedback!See how others have built the multiband fan
dipole using the new easy SRI technique:

09/2010."I wanted to provide some additional information about fan
dipoles.The SRI figures that you have posted are quite correct (I
believe that Donald Lee did the research for the US Army that created
those findings.) I do know from personal experience that the US Army
validated SRI's research findings with extensive field testing years ago.
I believe that for some unknown reason the Army always recommended 1 meter
(39.37") for end separation in fan dipoles."

"My own fan dipole
(80/40/20) built to the SRI specs needed no additional pruning in order to
achieve the appropriate resonant frequencies. Mine is an inverted V with
the feed point at 45' and the ends at 20'." Source -
anonymous.------------------------------------

Feedback
from another builder

"I
constructed it as per the stanford example. spaced the dipole elements 38
inches apart using pvc pipe.

The measurements I came
up with were different than the ones mentioned in the
article.

I built a
160/80/40.

The 160 was cut 4%
shorter with target freq at 1.9mhz,

The 80 and 40 both ended
up being 4% longer than the 468/freq formula.

Conclusion to achieve
target freq these are the formulas that worked for me.

Lowest freq antenna
468/freq X .96

Middle antenna 468/freq X
1.04

Lowest
antenna(highest freq antenna) 468/freq X 1.04

Hope this helps anyone
experimenting with the fan dipole.Details are as
follows.

CONSTRUCTING THE MULTIBAND DIPOLE:
(Older cut and try method)Here is a fairly simple and easy to build multi band
horizontal fan type dipole that can be constructed for all band operation
from 160 meters up thru 6 meters or even higher.In the drawing above,
it is shown for just four bands, 80 thru 10. One separate dipole for each
band needed. However you can build it to suit your own preferences by
using the standard formula for a dipole:468/freq mhz = total length
for each band. Use the formula for your desired center
frequency.

Each dipole length above
inREDis in feet and tenths of a foot for
the center of the General portion of each bandandis
derived from the above formula and should be
cutlongerfor swr
trimming. USE #12 TO #14 GAUGE COPPERWELD WIRE IF POSSIBLE or use what you
have on hand. The top most dipole must support the entire weight of the
antenna.

Start with your lowest (in frequency) band of operation as
the main (top) support for the entire setup. Cut it per the formula but
add a couple of feet on each end for tuning. Try to use a wire size that
will support the other dipoles.This is the main support for all the
other dipoles and must carry their weight.Cut a dipole for each band
of operation. (SEE EDITORS NOTE AT BOTTOM OF
ARTICLE)Cut each full length in
half....example: for the 10 meter length from the formula you get 16.1
feet for the total length. Cut it in half at about 8 feet per
side. Make sure you cut each length about a foot or more longer for swr
trimming and attaching to center and end insulators!If you are
building the four band dipole above, you should have 8 lengths of wire
scattered all over your work
area.

WARNING! DON'T DO
IT IN YOUR LIVING ROOM, THE XYL WILL NOT BE VERY HAPPY WITH YOU AND AFTER
SHE GETS FINISHED WITH THE QRM,,,, ALL YOUR ANTENNA BUILDING WILL HAVE TO
BE DONE FROM THE DOG'S HAM SHACK!

It is assumed that you have
your end support poles, trees, center and end insulators, pulleys all
ready to go before you start working on the actual
dipoles.A very important part of this
design is the installation of the pulleys (in yellow on drawing) on each
end attached to each side support.They are added to this design due to
the swr trimming process and make it very easy to pull the entire antenna
up and down while making the swr adjustments. Mount a suitable size
pulley on each end attached to your pole, trees, etc for the diameter of
cord or rope used to support the system.

Start your antenna
trimming with the top dipole.... attach your coax to the center insulator
leaving several inches of the center conductor and shield exposed. Each
half of each dipole will be connected to the coax center pigtail and the
shield separately. In other words, connect one side of the dipole to the
center conductor and the other side to the shield.Attach the other end
of each half of the longest wire to the support cord and run thru the
pulley on each end and pull the dipole up into the air between the end
supports. Check swr.Trim as needed with low power for lowest swr
possible, lower with pulleys, attach the next highest band dipole
electrically to the same point as the first dipole, raise it to operating
height, check swr, lower for trimming, up and down, up and
down.........due the same for all other dipoles for each higher band of
operation.When you are finished with the highest band of operation,
pull the entire system up with the pulleys and tie of at the bottom
securely.Make certain that the coax center conductor is attached to
one half of each dipole and the shield to the other half. All dipole ends
at center insulator are connected together.This may not be very clear
to the new antenna builder so please see the drawing below for the center
insulator arrangement.

(NOTE: IF USING THE NEWER CONSTRUCTION
METHOD MENTIONED ABOVE, INSURE PROPER SPACING OF ANTENNA LEGS AT
THIS CENTER INSULATOR!)

The white areas in the center support
drawing above are mechanical supports, clamps, wire ties or whatever your
genius can come up with to support the main (top wire) and the weight of
the coax.Remember, all the weight
of this antenna system is supported by the top wire.The connections
should be soldered and all should be sealed including coax end from water,
ice, snow etc.Use a 1:1 balun like the "Ugly
Balun" project page
on Hamuniverse.com close to the center before coax goes to your
rig.

For best performance get
it as high as possible and remember that since this is a dipole
arrangement, it will be somewhat bi-directional towards and away from you
as viewed in the drawing. (BROADSIDE)Remember that
all elements will interact with each other in the tuning process
and the final setup must be secured so the angle or distance between each
dipole does not change when blowing in the wind, etc.The angle or
distance between each dipole is not critical but the final spacing must be
maintained!It will take lots of work (trial and error) in getting each
dipole to the lowest SWR. Just keep TRYING.It should also be noted
that the antenna can be used in an inverted v fashion but remember the
spacing should be secure in the final operating position. Tune it as in
all the above instructions. You may use a tuner with this antenna
un-trimmed to save a lot of work but doing it correctly for best swr
without a tuner is always better!EXPERIMENT! EXPERIMENT!
EXPERIMENT!

Editors note:The multiband
fan dipole can be very difficult to tune for lowest swr in some
installations. There are many variables that will make tuning difficult.
Height above ground, sometimes the angle of each dipole relative to the
other dipoles, surroundings , etc. If you can get the swr to around 2 to 1
or lower for each band....don't worry too much about it.(see the newer construction method
above)

You might also consider using a good antenna tuner if
you are having major tuning problems. A 2:1 SWR or lower can be
handled by most builtin tuners in
radios.

You might also consider removing HF combinations such as
40/15 meters and 80/30 meters.For these cases, cut the element for the
lower frequency and let it servedouble duty at the odd
harmonic. In other words, cut the 40 meter element and let it serve
also as the 15 meter element which eliminates the 15 meter
section.Make sure that the distance between all dipole elements does
not change when tuning.They must be in a fixed position always with
some sort of spacer. In theory, we could fashion a four-wire antenna for
the 80, 40, 30, 20, 15 and 10-meter bands.

In practice, it may be
difficult to obtain a good match on all bands.Since the resonant
length of a given element in the presence of the others is not the same as
a dipole by itself, tuning can be a tedious and difficult procedure.
Adjust elements for resonance in order from lowest frequency to the
highest such as in an 80 40 20 10 combo.....start with 80 first.....then
go to next higher frequency dipole.Always cut each dipole a lot longer
than required for each band to make tuning easier.Trim as needed for
your operating frequency.All of these bandwidth, adjustment and
matching problems are easily solved with an antenna tuner at the
transmitter, feeding the antenna through 100 feet or less of RG-8
coax.

Please remember to send us feedback if you are using the
newer construction method or if you have any tips you would like to pass
along to others that make the multiband dipole easier or faster to
get set up! 73! Email to N4UJW at
Hamuniverse.com